This invention generally relates to fiber and waveguide optics, and more specifically, to optical interconnects in fiber optic transmissions.
Fiber optics and optical interconnects are used in a wide variety of applications. The use of optical fibers as a medium for transmission of digital data (including voice data) is becoming increasingly more common due to the high reliability and large bandwidth available with optical transmission systems. In addition, optical interconnects provide for a significant increase to the available bandwidth of board-level interconnects.
For example, short-reach interconnects over multi-mode optical fiber (MMF), on the order of 100 m, are widely used in computer systems, data centers, and campus networks. For these data links, the optical channel contributes relatively little signal degradation for 10-40 G data rates. In addition, generations of fiber with optimized modal bandwidth continue to be developed that minimize this primary source of signal degradation. The biggest challenge for short-reach interconnects is to produce active components, namely optical transmitters and receivers, that provide sufficient bandwidth to operate at high data rates (>20 Gb/s) while maintaining low power consumption to maximize the power efficiency, commonly expressed in mW/Gb/s or pJ/bit: the energy required to transmit a bit of information.
The typical approach to implementing an optical link is to separate the transmitter and receiver and design them separately. The transmitter (TX) is optimized to make its digital optical output as ideal as possible, while the receiver (RX) is designed to receive the transmitted optical signal and convert it to an electrical signal with high sensitivity (to operate with as little optical power as possible at a given bit-rate), and with minimum added jitter. Both TX and RX are designed to meet their specifications while consuming a minimum amount of power.
The most straightforward method for building optical TXs and RXs is to utilize high-speed analog drivers and receiver amplifiers that have sufficient bandwidth to faithfully convert the electrical signal to an optical signal and back again with minimal distortion. However, as data rates have increased beyond 10 Gb/s, it is difficult to realize the multimode optical devices (lasers and detectors) and amplifier circuits with sufficient raw bandwidth with acceptable power consumption. To achieve the highest data rates, the lasers must be operated at high current densities and the circuits typically consume high amounts of electrical power to deliver the required speed performance.